The present invention relates to an optical amplification apparatus and an optical transmission system capable of realizing long distance transmission with higher stability and reliability by compensating for a signal light distorted in an EDFA through Raman amplification so as to become the most adequate input power for the EDFA.
There has been a problem on increasing data traffic caused by increased number of callings in communications and a larger amount of contents data such as moving pictures under such situations as recent rapid penetration of the Internet and sharply increased accesses between intra corporate LANs. Therefore, a WDM (Wavelength Division Multiplexing) system is remarkably progressing and becoming rapidly widespread in order to prevent reduction of communication performance due to increased data traffic.
The WDM system has realized high-capacity transmission that is 100 times as high as conventional transmission through a line of fiber by superposing a plurality of optical signals on different wavelengths. Particularly, the existing WDM system uses an erbium-doped fiber amplifier (hereafter EDFA) to enable broadband long distance transmission. The EDFA mentioned here indicates an amplifier to which the principle as follows is applied. This principle is that light having a wavelength of 1550 nm-band as a transmission signal is amplified in a specified fiber when a pumping laser having a wavelength of 1480 nm or 980 nm is passed through the specified optical fiber obtained by doping an element such as erbium into the fiber.
FIG. 9 is a block diagram showing a schematic structure of the conventional WDM system. As shown in FIG. 9, the conventional WDM system has EDFAs (100, 110) in each predetermined zone on a transmission line 99 based on an optical fiber as a transmission medium. The signal light passing through the transmission line 99 is amplified by these EDFAs to maintain the lowest possible power required only for being recognized as information.
Each of the EDFAs (100, 110) generally comprises an erbium-doped fiber, pumping laser for pumping the erbium-doped fiber, optical isolator, and the optical filter (not shown).
The EDFAs (100, 110) for performing such multiple wavelength amplification are required to have a flat gain profile over a multiplexed wavelength band so that the degree of amplification is prevented from being different in each wavelength. That is, the EDFAs (100, 110) are desired to minimize a gain deviation of each signal light in the multiplexed wavelength band.
Therefore, generally, in the EDFAs (100, 110), gain specification is in many cases optimized by a gain equalization filer or the like so as to show the flattest gain profile with respect to a signal light having a specified signal light power. FIG. 10 shows a diagram for explaining a gain profile in the conventional EDFA. In FIG. 10, gain profiles in a case where the signal light power is xe2x88x9217 dBm and a case where it is xe2x88x9225 dBm are shown. Particularly, this EDFA is adjusted so that the uniform gain can be obtained over wavelengths of 1540 nm to 1580 nm when the signal light power of xe2x88x9217 dBm is received. On the other hand, when the signal light power of xe2x88x9225 dBm is received, a gain deviation on the short wavelength side is large as compared to the case where the signal light power of xe2x88x9217 dBm is received. Therefore, uniform gain cannot be obtained.
Accordingly, the WDM system using the EDFA is desired to design the power of the signal light to be input into the EDFA so that the gain profile of the EDFA becomes the flattest.
However, the WDM system for an ultra long distance such that the number of repeaters exceeds 100 has a problem that a gain band is narrowed because a gain deviation is accumulated as the number of repeating stages increase even if the gain deviation in the EDFA is a small amount.
FIG. 11A and FIG. 11B are diagrams for explaining the above-mentioned problems. FIG. 11A shows an output spectrum at an output port PA of the EDFA 100 in the first stage shown in FIG. 9, and FIG. 11B shows an output spectrum at an output port PB of the EDFA 110 in the following stage shown in FIG. 9. As shown in FIGS. 11A and 11B, even the signal light having the same information is output as the signal light having a different power distribution between the outputs of continuously disposed EDFAs. This is because the signal light power is not amplified perfectly flatly over multiple wavelengths due to the fine gain deviation, and in addition, the signal light is deviated from the most appropriate power by the gain deviation, so that the signal light cannot undergo amplification by a flat gain profile in the next EDFA.
The EDFA in particular cannot avoid production of ASE (Amplified Spontaneous Emission) noise. Therefore, as shown in FIG. 11A, the signal light spectrum together with the ASE component 120 undergoes amplification by the same gain profile. Accordingly, as shown in FIG. 11B, the ASE component 130 is also affected by the gain deviation.
The EDFA is a lumped amplifier in which parts pumping the optical signal are concentrated. Therefore, this lumped amplifier has such restriction that it undergoes propagation loss along an optical fiber as a transmission line leading to accumulation of noise, and non-linearity that may cause signal distortion and noise. Further, the EDFA enables optical amplification in a wavelength band defined by band gap energy of erbium. Therefore, the EDFA has difficulty in working on a wider band required for further multiplexing.
To solve the problem, a Raman amplifier has been focused on as an optical amplifier instead of the EDFA. The Raman amplifier is a distributed optical amplifier that does not require a specific fiber such as an erbium-doped fiber like the EDFA and uses an ordinary optical fiber for a transmission line as a gain medium.
However, a WDM system using this Raman amplifier also has problems as follows because at least two pumping light sources provided in the respective Raman amplifiers are always operated in constant output power.
(1) In Raman amplification, a transmission line through which a signal light is transmitted is used as a medium for amplification. Therefore, amplification characteristics of the medium depend on a type of transmission line (optical fiber). For example, an SMF (Single Mode optical Fiber) has an efficiency (Raman gain/pumping light power) by one-half as compared to that of a DSF (Dispersion Shifted Optical Fiber). Therefore, when the type of optical fiber forming the transmission line is changed while output power of the pumping light source is kept constant, the amplification characteristics of the transmission line are changed to be incapable of maintaining sufficient transmission quality if the characteristics remain changed.
(2) In Raman amplification, a transmission line through which a signal light is transmitted is used as a medium for amplification. Therefore, amplification characteristics of the medium also depend on transmission loss of the transmission line (optical fiber). Therefore, when the transmission loss of the optical fiber forming the transmission line fluctuates while output power of the pumping light source is kept constant, the amplification characteristics of the transmission line also fluctuate to be incapable of maintaining sufficient transmission quality. Further, when the transmission loss of the transmission line becomes larger, not only the signal light undergoes a large loss but also the pumping light undergoes a large loss, which makes the Raman gain decreased. Therefore, the fluctuation in the signal light power becomes larger than the fluctuation in the loss of the transmission line.
(3) When a wavelength multiplexed optical signal is subjected to Raman amplification, the amplification gain depends on the number of wavelengths (number of channels) of the signal light and optical power for each signal light in each wavelength. Therefore, when the number of channels of the signal light is increased or decreased while output power of the pumping light source is kept constant, the amplification characteristics fluctuate to be incapable of maintaining sufficient transmission quality.
According to the particularly recent studies, it is found that the most adequate system architecture can be built by not using the Raman amplifier as a single unit but by using it together with the EDFA. Therefore, the transmission capacity of this system can be expected to be improved by several times to ten times or more as compared to the system using only the EDFA. However, the WDM system using the Raman amplifier has not been established yet and is still in a stage of studying on how it is introduced.
It is an object of this invention is to provide an optical amplification apparatus and an optical transmission system capable of realizing long distance transmission with higher stability and reliability by flattening a signal light distorted over multiple wavelengths due to passing through EDFAs and a transmission line by Raman amplifiers.
In order to achieve the object, according to one aspect of this invention, a signal light having non-uniform power between wavelengths due to gain deviation can be corrected by Raman amplification so as to be flat.
According to another aspect of this invention, by providing an estimation unit and a gain profile determination unit in a Raman amplifier of a distributed amplification type, an incident signal light can be amplified so as to become a flat gain profile over a multiplexed wavelength.
According to still another aspect of this invention, a signal light having a flat signal light power obtained by suppressing a gain deviation to a minimum through Raman amplification based on distributed amplification can be input to a lumped amplifier.
Further, by providing an estimation unit and a gain profile determination unit in a Raman amplifier of a distributed amplification type, the signal light to be input to the lumped amplifier can be amplified so as to become a flat gain profile over the multiplexed wavelength.
According to still another aspect of this invention, a signal light whose gain deviation is suppressed to a minimum through Raman amplification based on backward pumping can be input to a lumped amplifier in the next stage.
Further, it is possible to compensate for distortion of the signal light due to gain deviation caused by an erbium-doped fiber amplifier.
According to still another aspect of this invention, by providing an estimation unit and a gain profile determination unit in a Raman amplifier of a distributed amplification type, a signal light to be propagated on a transmission line can be amplified so as to become a flat gain profile over a multiplexed wavelength.
Further, as a unit for transmitting a deviation of an estimated power distribution of the signal light from a first optical amplification apparatus to a second optical amplification apparatus, a system composed of an OSC transmitter and an OSC receiver can be utilized.
According to still another aspect of this invention, even if a type of optical fiber as a transmission medium of a signal light and also as a amplification medium of the signal light may be changed or transmission loss may fluctuate or even if the number of wavelengths of a transmitted signal light and light power of each signal light in each wavelength may fluctuate, preferred or the most adequate amplification characteristics can be always ensured and sufficient transmission quality can be maintained by changing the wavelengths of a pumping light and pumping light power according to their fluctuations.
Other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.